Blood drawing syringes comprise a cylindrical barrel having an open rearward end and a reduced diameter front end, or luer, for receiving the mounting collar of a needle. Inside the barrel a piston is movable lengthwise by an attached plunger stem which emerges from the open rear end of the barrel to be grasped by fingers. An example of such a syringe is shown in my U.S. Pat. No. 4,466,446 entitled Plunger Subassembly For Blood Gas Syringes.
To draw blood the plunger stem is first pulled rearwardly to withdraw the piston, providing the necessary volume in the forward end of the barrel for receiving and holding the amount of blood desired to be taken for the sample. The needle mounted on the syringe is then inserted into a blood vessel such as an artery. Arterial blood pressure will usually be sufficient to force the blood through the needle and into the barrel forward of the piston. In some cases, however, there is insufficient blood pressure to fill the barrel and aspiration is necessary in order to produce a low pressure in the forward part of the barrel sufficient to draw the blood. In order to create a lower pressure in the barrel of the syringe than exists in the blood vessel, the piston must be further withdrawn, expanding the volume without admitting air into the space. The prior art has developed two primary types of device to achieve this objective.
The first type employs a hollow or tubular plunger having a filter in its piston through which air can pass from the forward portion of the barrel into the interior of the plunger to be vented through an opening in the rear of the plunger. The filter is of the hydrophobic type which will allow the passage of gases but block the passage of liquid, such as blood. When pushing the plunger into the syringe barrel the open end of the plunger is allowed to remain open to allow the air forward of the piston to escape through the filter and the interior of the plunger tube. When the piston is to be withdrawn with the intent of creating a low pressure aspiration effect, the opening in the plunger must be closed with a thumb or cap in order to block air from moving forwardly through the interior of the plunger tube and through the filter back into the barrel.
If aspiration is found to be necessary after the needle is inserted, positioning a thumb or finger over the opening or locating and positioning the cap or plug to close the open end of the plunger, is awkward at best and difficult to accomplish without painfully gyrating the needle.
A second type of piston arrangement also utilizes a hydrophobic filter, but does not depend on a hollow plunger to convey the air from the forward side of the barrel. Various types of valving arrangements interior of the piston have been worked out but all suffer from one or more difficulties. One type employs a steel ball valve inside the piston, which, if allowed to descend in its channel for a period of time approximating fifteen seconds will seat and act as a block to air flow coming from the rear portion of the piston. The trouble with this arrangement is that the necessity for aspiration must be known in advance of inserting the needle in the blood vessel so that the syringe can be held in a vertical position for the required length of time to seat the ball valve. If the need for aspiration is discovered after needle insertion, normally at 45.degree., then the needle must first be withdrawn before positioning the syringe vertically to reorient the ball valve.
Another type of plunger tip allows air to pass out of or into the interior of the piston body through slits in the face or perimeter of the piston. Air enters the piston interior through spaces between the head of the plunger and the inside flanges of the piston body against which the head of the plunger bears. Such a system is shown in this inventor's U.S. Pat. No. 4,466,446. It is seen from an examination of the teaching of this patent that when the plunger is pulled out of the barrel the flanged head of the plunger bears tightly against the flanged interior of the piston bore, creating an air tight seal which does not allow air to enter the interior of the piston from the plunger side. Other generally similar syringes are disclosed in U.S. Pat. Nos. 2,882,899; 3,674,181; 3,147,753; 3,164,303; 3,874,382; 3,941,129 and 4,159,713.
The problem which is inherent with syringe pistons of the type disclosed in these patents arises from the very limited degree of aspiration which can be accomplished after needle insertion in cases where the need for aspiration is only then discovered. Normally the piston is positioned for the amount of blood desired to be taken prior to needle insertion. If, however, the blood pressure is sufficient to only partially fill the barrel space or body tissue debris clogs the needle bore and prevents the free flow of blood by arterial pressure, then the plunger must be withdrawn further to provide the necessary low pressure to aspirate the blood into the barrel. But withdrawing the piston from its first position limits the length of the stroke, thereby limiting the low pressure which may be created. In order to maximize the effectiveness of the aspiration the piston should preferably be pushed down in the barrel until it contacts the blood which is in the forward end of the barrel and then withdrawn, creating as large a low pressure as possible. This procedure cannot be done with state of the art pistons because forward movement of the plunger seals the air passage through the piston's interior, forcing the air in the forward part of the tube out through the needle. Pumping air into a blood vessel with this maneuver is not acceptable.
It is therefore the primary object of the present invention to provide a novel blood drawing syringe piston which may be preset in position within the syringe barrel for a given volume of blood to be drawn and which will pass the entrapped air through the piston as blood fills the syringe barrel.
A second object of the invention is to provide a novel blood drawing syringe piston which will accomplish greater aspiration efficiency by permitting the piston to be pushed forward without pumping air through the needle into the blood vessel and permitting the piston to be subsequently withdrawn without admitting air into the enlarging barrel chamber so as to create low pressure therein for aspiration purposes.